Structural Evolution of Nb/NbC Multilayer Coatings

Abstract:

Article Preview

The present communication is concerned with the interdiffusion kinetics and the interface
breakdown that take place in the Nb/NbC multilayer system as the result of thermal annealing in the
400-800oC temperature range. Within this temperature range carbon is the diffusing species. Carbon
diffuses from the carbide layer into the adjacent Nb layer, depleting its concentration within the carbide,
causing the nucleation and subsequent growth of an intermediate Nb2C layer and decreasing the width of
the original Nb layer. TEM examination of the cross-sections of the multilayer specimens provides data
regarding the evolution of the microstructure and, in particular, regarding the initial nucleation stage of
the newly formed Nb2C layer.

Abstract: The static constant corrosion tests on Fe-B eutectic alloy are investigated in liquid zinc at 500°C. The systematic observation and research of the corrosion layer are performed using SEM, TEM, XRD. The results show that corrosion resistance of Fe-B eutectic alloy in liquid zinc is perfect. Corrosion products are Fe3B and FeZn7. The corrosion layer is regular, the structure of which is hollow dendritic skeleton and the thickness of which increases with the corrosion time extending. Corrosion process is that the interdiffusion of ferro and zinc atomic results in corrosion layer formation and the corrosion layer ruptures and dissolves under the action of thermal stress and impact stress of liquid zinc. The process repeating leads to the failure of the matrix eventually.

Abstract: Diffusion treatments of TiAl-based alloys (49.1 at% Al) aluminum coated by thermal spray were carried out at the temperature range of 700°C-1100°C. The influence of the diffusion condition for the formation of intermetallic phases in the coating has been investigated. In the initial stage of diffusion treatment, TiAl3 was formed on the outermost surface by the diffusion between liquid aluminum and the substrate. In addition, an intermediate layer comprised of Ti2Al5 (at 1100°C), TiAl2 and Al diffused layer (Al-rich TiAl) was confirmed under the outermost layer. The maximum thickness of TiAl3 during the initial stage increases as the diffusion temperature decreases. In addition, the shape of TiAl3 layer was dependent on the diffusion temperature; the outermost layer without pores was confirmed at the temperature of 700°C. TiAl2 and Al-rich TiAl developed by solid-state diffusion from TiAl3 layer following a parabolic low. The activation energies for growth have been calculated to be 194 kJ/mol for TiAl2 and 292 kJ/mol for Al-rich TiAl.

Abstract: In this work, the thermal conductivity problem is solved for fourth layer plate at the heat flux action on one of surfaces. The interfaces between layers are assumed non ideal that leads to appearance of thermal resistances between layers. Additionally thermal resistances are inhomogeneous due to curved surfaces of materials. The existence of curved surfaces and porosity of layers are connected with the way of specimen manufacturing. Thermal conductivity coefficients of layers can by changed when their porosity changes. The problem is solved numerically. It was found that curved interfaces and thermal resistance affect process only at initial stage of the heating. Temperature gradients depend essentially on porosity and thickness of layers.